Breaking change in self-update function

[buildplan]

Since v0.34 flexible shebang check in the self-update function will break the backward compatibility for self-update, so a one-time manual update is required. Which can be done with:
curl -Lo restic-backup.sh "https://raw.githubusercontent.com/buildplan/restic-backup-script/main/restic-backup.sh" && chmod +x restic-backup.sh